The third Generation Partnership Project (3GPP) initiative called “License Assisted Access” (LAA) has the purposed to allow Long Term Evolution (LTE) device such as a User Equipment (UE) or a eNB base station to also operate in the unlicensed 5 GHz radio spectrum. The unlicensed 5 GHz spectrum is used as a complement to the licensed spectrum. Accordingly, devices connect in the licensed spectrum using a Primacy Cell (PCell) and use Carrier Aggregation (CA) to benefit from additional transmission capacity in the unlicensed spectrum using one or more Secondary Cells (SCell). To reduce the changes required for aggregating licensed and unlicensed spectrum, the LTE frame timing in the PCell is simultaneously used in the SCell.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, a so called Listen-Before-Talk (LBT) method is applied. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard. This standard is also known as “Wi-Fi.”
The regulations may vary from region to region. For example in Europe, the LBT procedure is under the scope of so-called a harmonized European Standard (EN) regulation also called EN 301.893 produced by European Telecommunications Standard Institute (ETSI). For LAA to operate in the 5 GHz spectrum the LAA LBT procedure should conform to requirements and minimum behaviors set forth in EN 301.893. However, additional system designs and steps are needed to ensure coexistence of Wi-Fi and LAA with EN 301.893 LBT procedures.
In the following a general description of the technologies involved in LAA is presented which include LTE where the spectrum is licensed, and a system employing the LBT procedure e.g. the WiFi or WLAN in order to understand the background of the embodiments herein.
LTE uses OFDM (Orthogonal Frequency Division Multiplexing) in the downlink and single-carrier FDMA (Frequency Division Multiple Access) in the uplink. The basic LTE downlink physical resource may be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing as the downlink and the same number of (Single Carrier) SC-FDMA symbols in the time domain as OFDM symbols in the downlink. The OFDM symbol is also shown including the Cyclic Prefix (CP) and an inter-subcarrier spacing of 15 kHz. A resource element is also indicated.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms as shown in FIG. 2. For normal CP, one subframe consists of 14 OFDM symbols. The duration of each symbol is approximately 71.4 μs.
Furthermore, the resource allocation in LTE is described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station or eNB transmits control information informing about which terminals (or UEs) data is transmitted to and upon which radio downlink resource blocks the data is transmitted, in the current downlink subframe. This control signaling usually is transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of e.g. the control information. A downlink system with CFI=3 OFDM symbols as control (control region) is illustrated in FIG. 3.
The reference symbols shown in FIG. 3 are known as Cell specific Reference Symbols (CRS) and are used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
In LTE there are channels known as the Physical Dedicated Control Channel (PDCCH) and the Enhanced PDCCH (EPDCCH).
Both the PDCCH/EPDCCH are used to carry downlink control information (DCI) such as scheduling decisions and power-control commands. The DCI includes:                Downlink scheduling assignments, including Physical Downlink Shared CHannel (PDSCH) resource indication, transport format, hybrid-ARQ information, and control information related to spatial multiplexing (if applicable). A downlink scheduling assignment also includes a command for power control of the Physical Uplink Control Channel (PUCCH) used for transmission of hybrid-ARQ acknowledgements in response to downlink scheduling assignments.        Uplink scheduling grants including a Physical Uplink Shared Channel (PUSCH) resource indication, a transport format, and hybrid-ARQ-related information. An uplink scheduling grant also includes a command for power control of the PUSCH.        Power-control commands for a set of terminals (UEs) as a complement to the commands included in the scheduling assignments/grants.        
A PDCCH/EPDCCH carries one DCI message containing one of the groups of information listed above. As multiple terminals (UEs) can be scheduled simultaneously, and each terminal can be scheduled on both downlink and uplink simultaneously, it is possible to transmit multiple scheduling messages within each subframe. Each scheduling message is transmitted on separate PDCCH/EPDCCH resources, and consequently there are typically multiple simultaneous PDCCH/EPDCCH transmissions within each subframe in each cell. Furthermore, to support different radio channel conditions, link adaptation can be used, where the code rate of the PDCCH/EPDCCH is selected by adapting the resource usage for the PDCCH/EPDCCH, to match the radio-channel conditions.
Further, in the LTE system, a UE is notified by the network of downlink data transmission by means of the PDCCH. Upon reception of a PDCCH in a subframe n, a UE (receiver in FIG. 4) is required to decode the corresponding physical downlink share channel (PDSCH) and is required to send ACK/NACK feedback in a subsequent subframe n+k. This is illustrated in FIG. 4.
The ACK/NACK feedback from the UE informs the eNodeB or eNB (transmitter in FIG. 4) whether the corresponding PDSCH was decoded correctly. When the eNodeB detects an ACK feedback, it can proceed to send new data blocks (new TX) to the UE. When a NACK is detected by the eNodeB, coded bits corresponding to the original data block will be retransmitted. When the retransmission (reTX) is based on repetition of previously sent coded bits, it is said to be operating in a chase combining HARQ protocol. When the retransmission contains coded bits unused in previous transmission attempts, it is said to be operating in an incremental redundancy HARQ protocol.
The ACK/NACK feedback is sent by the UE using one of the two possible approaches depending on whether the UE is simultaneously transmitting a physical uplink shared channel (PUSCH):
If the UE is not transmitting a PUSCH at the same time, the ACK/NACK feedback is sent via a physical uplink control channel (PUCCH).
If the UE is transmitting a PUSCH simultaneously, the ACK/NACK feedback is sent via the PUSCH.
LTE supports bandwidths larger than 20 MHz. One important requirement on LTE Rel-10 is to assure backward compatibility with LTE Release 8 (RL-8). This should also include spectrum compatibility. That would imply that an LTE Rel-10 carrier, wider than 20 MHz, should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In particular for early LTE Rel-10 deployments it can be expected that there will be a smaller number of LTE Rel-10-capable terminals compared to many LTE legacy terminals.
Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals (Rele 8 terminals) can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this would be by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple CCs, where the CC have, or at least the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 5. A CA-capable UE is assigned a primary cell (PCell) which is always activated, and one or more secondary cells (SCells) which may be activated or deactivated dynamically.
The number of aggregated Component Carries (CC) as well as the bandwidth of the individual CC may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case that the number of CCs is different. It is important to note that the number of CCs configured in a cell may be different from the number of CCs seen by a terminal: A terminal (e.g. UE) may for example support more downlink CCs than uplink CCs, even though the cell is configured with the same number of uplink and downlink CCs.
In addition, a feature of carrier aggregation is the ability to perform cross-carrier scheduling. This mechanism allows a (E)PDCCH on one CC to schedule data transmissions on another CC by means of a 3-bit Carrier Indicator Field (CIF) inserted at the beginning of the (E)PDCCH messages. For data transmissions on a given CC, a UE expects to receive scheduling messages on the (E)PDCCH on just one CC—either the same CC, or a different CC via cross-carrier scheduling; this mapping from (E)PDCCH to PDSCH is also configured semi-statically.
As previously described, in LAA systems a sharing of spectrum is performed wherein LTE which operates at a licensed spectrum and WLAN or WiFi operates at an unlicensed spectrum. In the following the WLAN or WiFi system is briefly described and particularly how the channel is accessed in WLAN systems.
In typical deployments of WLAN, Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is used for medium access. This means that the channel is sensed to perform a Clear Channel Assessment (CCA), and a transmission is initiated only if the channel is declared as Idle. In case the channel is declared as Busy, the transmission is essentially deferred until the channel is deemed to be idle. When the range of several Access Points (APs) using the same frequency overlap, this means that all transmissions related to one AP might be deferred in case a transmission on the same frequency to or from another AP which is within range can be detected. Effectively, this means that if several APs are within range, they will have to share the channel in time, and the throughput for the individual APs may be severely degraded. A general illustration of the Listen-Before-Talk (LBT) mechanism or procedure is shown in FIG. 6.
After a WLAN station A transmits a data frame to a station B, station B shall transmit the ACK frame back to station A with a delay of 16 μs. Such an ACK frame is transmitted by station B without performing a LBT operation. To prevent another station interfering with such an ACK frame transmission, a station shall defer for a duration of 34 μs (referred to as DIFS) after the channel is observed to be occupied before assessing again whether the channel is occupied.
Therefore, a station that wishes to transmit first performs a CCA by sensing the medium for a fixed duration DIFS. If the medium is idle then the station assumes that it may take ownership of the medium and begin a frame exchange sequence. If the medium is busy, the station waits for the medium to go idle, defers for DIFS, and waits for a further random backoff period.
To further prevent a station from occupying the channel continuously and thereby prevent other stations from accessing the channel, it is required for a station wishing to transmit again after a transmission is completed to perform a random backoff.
The PIFS is used to gain priority access to the medium, and is shorter than the DIFS duration. Among other cases, it can be used by stations operating under PCF, to transmit Beacon Frames with priority. At the nominal beginning of each Contention-Free Period (CFP), the AP shall sense the medium. When the medium is determined to be idle for one PIFS period (generally 25 μs), the AP shall transmit a Beacon frame containing the CF Parameter Set element and a delivery traffic indication message element.
It should be mentioned that when the medium becomes available, multiple WLAN stations may be ready to transmit, which can result in collision. To reduce collisions, stations intending to transmit select a random backoff counter and defer for that number of slot channel idle times. The random backoff counter is selected as a random integer drawn from a uniform distribution over the interval of [0, CW]. The default size of the random backoff contention window, CWmin, is set in the IEEE specifications. Note that, collisions may still happen even with this random backoff protocol when they are many stations contending for the channel access. Hence, to reduce continuous collisions, the backoff contention window size CW is doubled whenever the station detects a collision of its transmission up to a limit, CWmax, also set in the IEEE specifications. When a station succeeds in a transmission without collision, it resets its random backoff contention window size back to the default value CWmin.
It should also be mentioned that for a device not utilizing the Wi-Fi (WLAN) protocol, EN 301.893 provides the following requirements and minimum behavior for the load-based clear channel assessment.
1) Before a transmission or a burst of transmissions on an Operating Channel, the equipment (AP or UE) shall perform a CCA check using “energy detect”. The equipment shall observe the Operating Channel(s) for the duration of the CCA observation time which shall be not less than 20 μs. The CCA observation time used by the equipment shall be declared by the manufacturer. The Operating Channel shall be considered occupied if the energy level in the channel exceeds the threshold corresponding to the power level given in point 5 below. If the equipment finds the channel to be clear, it may transmit immediately (see point 3 below).
2) If the equipment finds an Operating Channel occupied, it shall not transmit in that channel. The equipment shall perform an Extended CCA check in which the Operating Channel is observed for the duration of a random factor N multiplied by the CCA observation time. N defines the number of clear idle slots resulting in a total Idle Period that need to be observed before initiation of the transmission. The value of N shall be randomly selected in the range 1 . . . q every time an Extended CCA is required and the value stored in a counter. The value of q is selected by the manufacturer in the range 4 . . . 32. This selected value shall be declared by the manufacturer. The counter is decremented every time a CCA slot is considered to be “unoccupied”. When the counter reaches zero, the equipment may transmit.
3) The total time that an equipment makes use of an Operating Channel is the Maximum Channel Occupancy Time which shall be less than (13/32)×q ms, with q as defined in point 2 above, after which the device shall perform the Extended CCA described in point 2 above.
4) The equipment, upon correct reception of a packet which was intended for this equipment, can skip CCA and immediately proceed with the transmission of management and control frames (e.g. ACK and Block ACK frames). A consecutive sequence of transmissions by the equipment, without it performing a new CCA, shall not exceed the Maximum Channel Occupancy Time.
NOTE: For the purpose of multi-cast, the ACK transmissions (associated with the same data packet) of the individual devices are allowed to take place in a sequence.
5) The energy detection threshold for the CCA shall be proportional to the maximum transmit power (PH) of the transmitter: for a 23 dBm e.i.r.p. transmitter the CCA threshold level (TL) shall be equal or lower than −73 dBm/MHz at the input to the receiver (assuming a 0 dBi receive antenna). For other transmit power levels, the CCA threshold level TL shall be calculated using the formula: TL=−73 dBm/MHz+23−PH (assuming a 0 dBi receive antenna and PH specified in dBm e.i.r.p.).
An example of the LBT mechanism in EN 301.893 is depicted in FIG. 7.
Regarding LAA systems, up to now, the spectrum used by LTE is dedicated to LTE. This has the advantage that an LTE system does not need to care about coexistence with other non-3GPP radio access technologies in the same spectrum and spectrum efficiency can be maximized. However, the spectrum allocated to LTE is limited which cannot meet the ever increasing demand for larger throughput from applications/services. Therefore, a new study item has been initiated in 3GPP on extending LTE to exploit unlicensed spectrum in addition to licensed spectrum.
With LAA to unlicensed spectrum, as shown in FIG. 8, a UE is connected to a PCell operating in the licensed spectrum and one or more SCells operating in the unlicensed spectrum. In this application we denote a SCell in unlicensed spectrum as LAA secondary cell (LAA SCell). The LAA SCell may operate in DL-only mode or operate with both UL and DL traffic. Furthermore, in future scenarios the LTE nodes may operate in standalone mode in license-exempt channels without assistance from a licensed cell. Unlicensed spectrum may, by definition, be simultaneously used by multiple different technologies. Therefore, LAA as described above may coexists with other systems such as IEEE 802.11 (Wi-Fi or WLAN).
To coexist fairly with the Wi-Fi (WLAN) system, transmission on the SCell shall conform to LBT protocols in order to avoid collisions and causing severe interference to on-going transmissions. This includes both performing LBT before commencing transmissions, and limiting the maximum duration of a single transmission burst. A single transmission burst refers to a transmission by a node performed after a successful channel contention. The maximum transmission burst duration is country-specific and/or region-specific. For example, the maximum burst duration is 4 ms in Japan and 13 ms in Europe according to EN 301.893. An example in the context of LAA using carrier aggregation and LBT is shown in FIG. 9 with different examples for the duration of a transmission burst on the LAA SCell constrained by a maximum allowed transmission duration of 4 ms as an example.
The basic LAA coexistence protocols with fixed random backoff contention window sizes, such as that specified in ETSI EN 301.893, may handle networks with small or moderate number of nodes contending for channel access. Additional measures may be needed to handle cases when a large number of nodes are present in networks operating on the same channels.
The existing random backoff contention window protocol is based on the reception of a single ARQ feedback value (ACK/NACK) that is received after the transmission of a burst of data. In the case of LTE, a first hybrid ARQ (HARQ) protocol is followed instead of a simple ARQ protocol. Thus, multiple retransmissions based on HARQ feedback may be needed before a single ARQ feedback value at the higher layer is generated.
Further, multiple UEs may communicate with an eNB in a single subframe. In addition, a single LAA transmission may be comprised of multiple subframes. Further, a transmission to or from a single UE may have multiple HARQ feedback values. This is the case when e.g. a transmission is a multi-codeword transmission. Thus, there are multiple ways in which multiple feedback values may be received corresponding to a single transmission burst following a successful channel contention. The existing random backoff contention window protocol is not suitable to deal with HARQ feedbacks.
It should also be mentioned that a feature of LTE is that the HARQ feedback is only available after a delay of a fixed predetermined time e.g. 4 ms which corresponds to multiple subframes, whereas in other systems it is assumed that the feedback is available after a very short time interval after the transmission ends and this very short time interval may be shorter that the above delay defined in LTE. These systems do not effectively deal with a system like LTE where the feedback delay is much larger.